1. Field of the Invention
The present invention relates to a magneto-optical recording disk of an Erasable Direct Read After Write (E-DRAW) type, and more particularly to a magneto-optical recording disk comprising a thin film formed of an amorphous alloy essentially including rare earth and transition metals, which alloy is a magneto-optical recording material
2. Description of the Prior Art
In prior art techniques for making a magneto-optical recording disk, it is known that a rare earth--transition metal alloy layer formed under pre-selected conditions has an amorphous structure and has a uniaxial magnetic anisotropy i.e. the property of being readily magnetized in a vertical direction to the layer. Such a layer is effective for rotating the plane of vibration of an incident light beam in different directions in dependence upon the magnetized direction. This magneto-optical property is exploited in various magneto-optical recording disks each having a magneto-optical layer laminated with other thin films which have been recently developed. Since such a magneto-optical recording layer is apt to deteriorate due to oxidation, it is sandwiched between a pair of protective layers made of dielectric such as silicon nitride (SiN) or zinc sulfide (ZnS). These protective layers function as a barrier to the humidity passing through the substrate to protect the magneto-optical recording layer. Such layers also conduct a magneto-optical effect to enhance the Kerr rotation angle due to the multi-reflection of the laser beam therein.
A conventional construction of the magneto-optical recording disk is shown in FIG. 1, for example.
Such a magneto-optical recording disk comprises a transparent substrate 1 made of polycarbonate resin etc. on which a first protective layer 2 with the function of enhancing the Kerr-effect, a magneto-optical recording layer 3 made of TbFeCO (terbium-iron-cobalt) ternary alloy etc. and a second protective layer 4 made of dielectric material are disposed in such order as listed. The magneto-optical recording disk may be so constructed that a back plate 7 is adhered via an adhesive layer 6 on the second protective layer 4. A laser beam 8 for recording or reading is irradiated from the substrate 1 side.
The recording and reading of data in the magneto-optical recording disk is performed in the following steps. At first, the amorphous alloy thin file, i.e., the magneto-optical recording layer is uniformly magnetized in a direction vertical to the major surface of the disk. A laser beam is focused onto a small spot on the amorphous alloy thin layer to partly heat the small spot portion so as to raise the temperature of the small spot above the Curie temperature or the compensation temperature thereof while applying a bias magnetic field to the small spot in the direction opposite to the initial magnetization. In the small spot, the orientation of magnetization is reversed by the thermal demagnetization and the inversion of the magnetic pole. In this way, the inverted magnetic domains are formed as small spots in the film plane which has been uniformly magnetized. Secondly, a polarized laser beam is incident to a track made of a train of the inverted magnetic domains. The reading of data on the film plane takes place by utilizing the changes of a rotation of the optic axis and an elliptic ratio of the laser beam which are due to the Kerr effect on the surface of magneto-optical recording layer 4 and the Faraday effect in the Kerr-effect enhancing layer 3. On the basis of these changes, it can be determined whether or not the inverted magnetic domain exists. In this way, the recording and reading of data is performed by assigning the binary "1" and "0" signals to the magnetizing directions.
On the other hand, the conventional protective layers are made of polycrystalline dielectric material. It is a problem that the properties of the magneto-optical recording disk such as the carrier to noise (C/N) ratio becomes low because the irradiated laser beam is scattered by the micro crystalline structure of such protective layers.